The use of transparent material utilizing thermoplastic resin as a structural component is well known. Products made from many such thermoplastic resins, particularly polycarbonate resin, exhibit such properties as toughness, high transparency, good dimensional stability, excellent creep resistance, excellent thermal stability and the like. While thermoplastic resins, for example polycarbonate resin, are easily fabricated into the desired shape and have excellent physical and chemical properties, their scratch and abrasion resistance is relatively low.
In order to overcome low scratch and abrasion resistance, various known coatings have been recommended for use on non opaque plastics. Such coatings believed to exhibit the best degree of abrasion resistance are comprised of at least one silicone-containing compound. Especially preferred among the silicone containing coatings are those which employ the condensation product of a functional silane.
U.S. Pat. Nos. 3,451,838, 3,986,997 and 4,027,073 disclose organopolysiloxane coatings, compositions, and techniques for the application of such to polycarbonate substrate.
U.S. Pat. No. 3,707,397 describes a process for providing a hard coating on, inter alia, polycarbonate articles, said process including priming the polycarbonate surface with an adhesion promoting thermosettable acrylic and applying an organopolysiloxane onto the primed surface.
U.S. Pat. No. 4,207,357 describes a method for coating a polycarbonate article with a scratch-, mar-, abrasion-, and solvent-resistant colloidal silica filled organopolysiloxane coating. The method includes (i) applying and curing a primer composition of a thermosettable acrylic polymer and hydroxy ether and, (ii) applying and curing a hardcoat composition containing a colloidal silica filled organopolysiloxane. U.S. Pat. Nos. 4,242,381, 4,284,685, 4,308,317, and 4,353,959 describe a similar method and coating. Each of these patents disclose a thermoplastic or thermoset acrylic primer layer with an organopolysiloxane hardcoat.
The principle problem associated with the organopolysiloxane hardcoats is that of adhesion to the thermoplastic substrate. Acrylic primers, both thermoset and thermoplastic have been commonly used with good results as a primer layer to improve adhesion between hardcoats and substrates. However, even with the use of the acrylic primers, adhesion can often be erratic, leading to spot failures in the surface hardcoat. Spot failures result in degradation of, or damage to, the thermoplastic surface surrounding the area of the failure, particularly in abrasive or otherwise hostile conditions. Further, in spite of the good results obtained with the use of the acrylic primer, the long term effects of weathering will eventually result in adhesion failure.
Thus, it is desirable to lessen the erratic nature of the adhesion of organopolysiloxane hardcoats to the thermoplastic substrate while at the same time increasing the overall adhesion.
Therefore, it is an object of the present invention to provide an acrylic primer layer having increased levels of adhesion to the organopolysiloxane hardcoat.
It is another object of the present invention to provide a blended acrylic primer layer containing organopolysiloxane constituent.
It is still another object of the present invention to provide transparent thermoplastic substrates, especially polycarbonate substrates, having more firmly and consistently adhered thereto a protective organopolysiloxane hardcoat.